This invention relates to a permanent magnet type stepping motor, and more particularly to a permanent magnet type stepping motor adapted to exhibit increased rotational angle accuracy and static angle accuracy.
A typical two-phase permanent magnet type stepping motor which has been conventionally known in the art is constructed in such a manner as shown in FIGS. 8 and 9. In FIG. 8, reference numeral 1 designates a rotor including a cylindrical permanent magnet element 7 securely mounted on a cylindrical bushing 5 fixed on a revolving shaft 3. The revolving shaft 3 is supported by two bearings (not shown). The permanent magnet element 7 is constructed into a cylindrical configuration so as to permit n (n: a positive integer of 4 or more) N magnetic poles and n S magnetic poles to alternately appear at equal pitches thereon in a circumferential direction of the revolving shaft 3. The conventional stepping motor also includes a stator 9 arranged on an outer periphery of the rotor 1 while being mounted in a housing (not shown). The stator 9 includes a first claw pole type yoke unit 11 and a second claw pole type yoke unit 25. The first and second claw pole type yoke units 11 and 25 are juxtaposed to each other in an axial direction of the revolving shaft 3. The first and second claw pole type yoke units 11 and 25 are shifted from each other by a distance one quarter (1/4) as large as predetermined pitches P between the magnetic poles of the permanent magnet element 7 in the circumferential direction. The first claw pole type yoke unit 11 includes a first yoke 13 and a second yoke 17 respectively including n pole teeth 15 and 19 arranged opposite to the permanent magnet element 7 at a predetermined interval in a radial direction of the revolving shaft and positioned at predetermined pitches P in the circumferential direction. The first claw pole type yoke unit 11 also includes an exciting winding 21 constructed so as to permit the n pole teeth 15 of the first yoke 13 and the n pole teeth 19 of the second yoke 17 to be excited at polarities different from each other. The exciting winding 21 is wound on a bobbin 23.
The second claw pole yoke unit 25 likewise includes a first yoke 27 and a second yoke 31 respectively including n pole teeth 29 and 33 arranged opposite to the permanent magnet element 7 at a predetermined interval in the radial direction of the revolving shaft and positioned at predetermined pitches P in the circumferential direction. The second claw pole type yoke unit 25 also includes an exciting winding 35 constructed so as to permit the n pole teeth of the first yoke 27 and the n pole teeth 33 of the second yoke 31 to be excited at polarities different from each other. The exciting winding 35 is likewise wound on a bobbin 37.
In the conventional permanent magnet type stepping motor thus constructed, the pitch P between each adjacent two of the pole teeth 15, a pitch P between each adjacent two of the pole teeth 29, a pitch P between each adjacent two of the pole teeth 33 and the above-described pitch P between each adjacent two of the pole teeth of the permanent magnet element 7 are typically set to be identical with each other.
Unfortunately, the conventional permanent magnet type stepping motor thus constructed is deteriorated in rotational angle accuracy and static angle accuracy as compared with a hybrid type stepping motor. In order to eliminate such a problem, a variety of techniques were proposed as disclosed in, for example, Japanese Patent Application Laid-Open Publication No. 245929/1995, Japanese Patent Application Laid-Open Publication No. 245930/1995 and Japanese Patent Application Laid-Open Publication No. 127024/1998. The techniques proposed are generally constructed so as to vary a configuration of pole teeth or shift one of two yokes from the other yoke at a predetermined pitch in a circumferential direction of a revolving shaft, to thereby reduce a detent torque or decrease or remove a specific harmonic component contained in a waveform of induced electromotive force, resulting in eliminating the above-described problem. In the prior art, pitches between pole teeth of each of yokes of claw pole type yoke units are rendered constant.
As described above, those skilled in the art attempted to solve the above-described problem while rendering the pitches between the pole teeth of each of the yokes of the claw pole type yoke units constant. However, this fails to increase rotational angle accuracy and static angle accuracy.